DRM requires that the end user's software has the key to decrypt the DRM. Open Source DRM would require that the location and specification of the key are readily available, along with the exact decryption algorithm.

Open Source DRM will remain 'secure' for a very, very short time.

The thing is though, the key could be anything. OpenPGP is completely opensource, and operates on roughly the same principles as DRM, and is considered one of the most secure encryption methods available to the public. You don't hear about that getting cracked. It is not as simple as "the key must be stored here, so copy that and you're freely available to open it". And hell, even if you go based on your methods, that isn't any more secure than Adobe ADEPT or any of that stuff.